On the subject of its colour, lots of caterpillars turn brown when the time for pupation is approaching. I can't guarantee that that's what is happening here but if the caterpillar is well grown then it probably is.

Re its ID, yes I think it is a puss moth caterpillar though you have to be careful not confuse them with those of kitten moths (sallow kitten etc.).

Yep, these are pre-pupatory colours. As the leaves of their foodplant turn orange/brown verdant green is no longer a good colour to be if you wish to avoid predators. By the time they wander off to pupate they are almost purple in colour.

Super caterpillars - common enough but you can never guarantee finding them if you went hunting for them.